Model of an electric chiller, based on the model by Hydeman et al. (2002) that has been developed in the CoolTools project and that is implemented in EnergyPlus as the model Chiller:Electric:ReformulatedEIR. This empirical model is similar to Buildings.Fluid.Chillers.ElectricEIR. The difference is that to compute the performance, this model uses the condenser leaving temperature instead of the entering temperature, and it uses a bicubic polynomial to compute the part load performance.

This model uses three functions to predict capacity and power consumption:

These curves are stored in the data record per and are available from Buildings.Fluid.Chillers.Data.ElectricReformulatedEIR. Additional performance curves can be developed using two available techniques (Hydeman and Gillespie, 2002). The first technique is called the Least-squares Linear Regression method and is used when sufficient performance data exist to employ standard least-square linear regression techniques. The second technique is called Reference Curve Method and is used when insufficient performance data exist to apply linear regression techniques. A detailed description of both techniques can be found in Hydeman and Gillespie (2002).

The model takes as an input the set point for the leaving chilled water temperature, which is met if the chiller has sufficient capacity. Thus, the model has a built-in, ideal temperature control. The model has three tests on the part load ratio and the cycling ratio:

  1. The test
      PLR1 =min(QEva_flow_set/QEva_flow_ava, per.PLRMax);
    ensures that the chiller capacity does not exceed the chiller capacity specified by the parameter per.PLRMax.
  2. The test
      CR = min(PLR1/per.PRLMin, 1.0);
    computes a cycling ratio. This ratio expresses the fraction of time that a chiller would run if it were to cycle because its load is smaller than the minimal load at which it can operate. Note that this model continuously operates even if the part load ratio is below the minimum part load ratio. Its leaving evaporator and condenser temperature can therefore be considered as an average temperature between the modes where the compressor is off and on.
  3. The test
      PLR2 = max(per.PLRMinUnl, PLR1);
    computes the part load ratio of the compressor. The assumption is that for a part load ratio below per.PLRMinUnl, the chiller uses hot gas bypass to reduce the capacity, while the compressor power draw does not change.

The electric power only contains the power for the compressor, but not any power for pumps or fans.

The model can be parametrized to compute a transient or steady-state response. The transient response of the boiler is computed using a first order differential equation for the evaporator and condenser fluid volumes. The chiller outlet temperatures are equal to the temperatures of these lumped volumes.



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